Acetylcholinesterases from Musca domestica and Drosophila melanogaster brain are linked to membranes by a glycophospholipid anchor sensitive to an endogenous phospholipase

Characterization of molecular and kinetic properties of two acetylcholinesterases from the Colorado potato beetle, Leptinotarsa decemlineata

The molecular and biochemical properties of two acetylcholinesterases (LdAChE1 and LdAChE2) from the Colorado potato beetle, Leptinotarsa decemlineata, were investigated in this study. Polyacrylamide gel electrophoresis in conjunction with western blotting with LdAChE1- or LdAChE2-specific antibodies suggested that LdAChE1 exists in a soluble form, whereas LdAChE2 exists in both soluble and amphiphilic forms with a glycophosphatidylinositol anchor. Both LdAChEs exist as homodimers with each monomer connected with a disulfide bond. LdAChE1 was the most highly expressed in the thorax followed by the head, leg, and abdomen, whereas LdAChE2 was the most highly expressed in the head, followed by the thorax, leg, and abdomen. The overall expression levels of LdAChE1, however, were higher than those of LdAChE2 in all examined tissues. Kinetic analysis using recombinant LdAChE1 and LdAChE2 showed that LdAChE2 has a 4.8-fold higher catalytic efficiency toward acetylthiocholine iodide compared to LdAChE1. LdAChE2 was more sensitive to organophosphorus and carbamate insecticides than LdAChE1. The addition of irreversibly phosphorylated LdAChE1 via paraoxon titration significantly reduced LdAChE2 inhibition by insecticides and glycoalkaloids, suggesting a sequestration role of soluble LdAChE1 in the chemical defense against xenobiotics. Taken together, LdAChE2 may be the main enzyme for synaptic transmission, thus serving as a toxicologically more relevant target, whereas the soluble LdAChE1 may function as a bioscavenger.

Nematicidal activity of tirotundin and parthenolide isolated from Tithonia diversifolia and Chrysanthemum parthenium

Acetylcholinesterase (AChE) is an enzyme that catalyzes acetylcholine into choline and acetic acid. Conventional pesticides, including organophosphates and carbamates target and inhibit the activity of AChE. To obtain more pesticide precursors that meet the safety requirements, more than 200 compounds were screened. Tirotundin and parthenolide identified as potential neurotoxins to nematodes were isolated from Tithonia diversifolia and Chrysanthemum parthenium, respectively. Their IC₅₀ values were 6.89 ± 0.30 and 5.51 ± 0.23 μg/mL, respectively against the AChE isolated from Caenorhabditis elegans. AChE was inhibited in a dose-dependent manner using the two compounds. And the Lineweaver-Burk and Dixon plots indicated that tirotundin and parthenolide were reversible inhibitors against AChE, both inhibiting AChE in a mixed-type competitive manner and demonstrating these compounds may possess dual binding site AChE inhibitors. LC₅₀ values of tirotundin and parthenolide against C. elegans were 9.16 ± 0.21 and 7.23 ± 0.48 μg/mL, respectively. These results provide a certain theoretical basis for the development and utilization of novel pesticides.

In silico annotation of unreviewed acetylcholinesterase (AChE) in some lepidopteran insect pest species reveals the causes of insecticide resistance

Lepidoptera is the second most diverse insect order outnumbered only by the Coeleptera. Acetylcholinesterase (AChE) is the major target site for insecticides. Extensive use of insecticides, to inhibit the function of this enzyme, have resulted in the development of insecticide resistance. Complete knowledge of the target proteins is very important to know the cause of resistance. Computational annotation of insect acetylcholinesterase can be helpful for the characterization of this important protein. Acetylcholinesterase of fourteen lepidopteran insect pest species was annotated by using different bioinformatics tools. AChE in all the species was hydrophilic and thermostable. All the species showed lower values for instability index except L. orbonalis, S. exigua and T. absoluta. Highest percentage of Arg, Asp, Asn, Gln and Cys were recorded in P. rapae. High percentage of Cys and Gln might be reason for insecticide resistance development in P. rapae. Phylogenetic analysis revealed the AChE in T. absoluta, L. orbonalis and S. exigua are closely related and emerged from same primary branch. Three functional motifs were predicted in eleven species while only two were found in L. orbonalis, S. exigua and T. absoluta. AChE in eleven species followed secretory pathway and have signal peptides. No signal peptides were predicted for S. exigua, L. orbonalis and T. absoluta and follow non secretory pathway. Arginine methylation and cysteine palmotylation was found in all species except S. exigua, L. orbonalis and T. absoluta. Glycosylphosphatidylinositol (GPI) anchor was predicted in only nine species.

Acetylcholinesterases from Leaf-Cutting ant Atta sexdens: Purification, Characterization, and Capillary Reactors for On-Flow Assays

Acetylcholinesterase (AChE) is responsible for catalyzing the hydrolysis of the neurotransmitter acetylcholine (ACh) leading to acetate and choline (Ch) release. The inhibition of AChE produces a generalized synaptic collapse that can lead to insect death. Herein we report for the first time the isolation of two AChEs from Atta sexdens which were purified by sulphate ammonium precipitation followed by ion exchange chromatography. AsAChE-A and AsAChE-B enzymes have optimum pH of 9.5 and 9.0 and higher activities in 30/50°C and 20°C, respectively, using acetylthiocholine (ATCh) as substrate. Immobilized capillary enzyme reactors (ICERs) were obtained for both enzymes (AsAChE-A-ICER and AsAChE-B-ICER) and their activities were measured by LC-MS/MS through hydrolysis product quantification of the natural substrate ACh. The comparison of activities by LC-MS/MS of both AChEs using ACh as substrate showed that AsAChE-B (free or immobilized) had the highest affinity. The inverse result was observed when the colorimetric assay (Elman method) was used for ATCh as substrate. Moreover, by mass spectrometry and phylogenetic studies, AsAChE-A and AsAChE-B were classified as belonging to AChE-2 and AChE-1 classes, respectively.

Characterization of the Fifth Putative Acetylcholinesterase in the Wolf Spider, Pardosa pseudoannulata

Background: Acetylcholinesterase (AChE) is an important neurotransmitter hydrolase in invertebrate and vertebrate nervous systems. The number of AChEs is various among invertebrate species, with different functions including the 'classical' role in terminating synaptic transmission and other 'non-classical' roles. Methods: Using rapid amplification of cDNA ends (RACE) technology, a new putative AChE-encoding gene was cloned from Pardosa pseudoannulata, an important predatory natural enemy. Sequence analysis and in vitro expression were employed to determine the structural features and biochemical properties of this putative AChE. Results: The cloned AChE contained the most conserved motifs of AChEs family and was clearly clustered with Arachnida AChEs. Determination of biochemical properties revealed that the recombinant enzyme had the obvious preference for the substrate ATC (acetylthiocholine iodide) versus BTC (butyrylthiocholine iodide). The AChE was highly sensitive to AChE-specific inhibitor BW284C51, but not butyrylcholinesterase-specific inhibitor tetraisopropyl pyrophosphoramide (ISO-OMPA). Based on these results, we concluded that a new AChE was identified from P. pseudoannulata and denoted as PpAChE5. Conclusion: Here we report the identification of a new AChE from P. pseudoannulata and increased the AChE number to five in this species. Although PpAChE5 had the biggest Vmax value among five identified AChEs, it showed relatively low affinity with ATC. Similar sensitivity to test insecticides indicated that this AChE might serve as the target for both organophosphorus and carbamate insecticides.

Expression of acetylcholinesterase 1 is associated with brood rearing status in the honey bee, Apis mellifera

Acetylcholinesterase 1 (AmAChE1) of the honey bee, Apis mellifera, has been suggested to have non-neuronal functions. A systematic expression profiling of AmAChE1 over a year-long cycle on a monthly basis revealed that AmAChE1 was predominantly expressed in both head and abdomen during the winter months and was moderately expressed during the rainy summer months. Interestingly, AmAChE1 expression was inhibited when bees were stimulated for brood rearing by placing overwintering beehives in strawberry greenhouses with a pollen diet, whereas it resumed when the beehives were moved back to the cold field, thereby suppressing brood rearing. In early spring, pollen diet supplementation accelerated the induction of brood-rearing activity and the inhibition of AmAChE1 expression. When active beehives were placed in a screen tent in late spring, thereby artificially suppressing brood-rearing activity, AmAChE1 was highly expressed. In contrast, AmAChE1 expression was inhibited when beehives were allowed to restore brood rearing by removing the screen, supporting the hypothesis that brood rearing status is a main factor in the regulation of AmAChE1 expression. Since brood rearing status is influenced by various stress factors, including temperature and diet shortage, our finding discreetly suggests that AmAChE1 is likely involved in the stress response or stress management.

Identification and Biochemical Properties of Two New Acetylcholinesterases in the Pond Wolf Spider (Pardosa pseudoannulata)

Acetylcholinesterase (AChE), an important neurotransmitter hydrolase in both invertebrates and vertebrates, is targeted by organophosphorus and carbamate insecticides. In this study, two new AChEs were identified in the pond wolf spider Pardosa pseudoannulata, an important predatory natural enemy of several insect pests. In total, four AChEs were found in P. pseudoannulata (including two AChEs previously identified in our laboratory). The new putative AChEs PpAChE3 and PpAChE4 contain most of the common features of the AChE family, including cysteine residues, choline binding sites, the conserved sequence 'FGESAG' and conserved aromatic residues but with a catalytic triad of 'SDH' rather than 'SEH'. Recombinant enzymes expressed in Sf9 cells showed significant differences in biochemical properties compared to other AChEs, such as the optimal pH, substrate specificity, and catalytic efficiency. Among three test substrates, PpAChE1, PpAChE3 and PpAChE4 showed the highest catalytic efficiency (Vmax/KM) for ATC (acetylthiocholine iodide), with PpAChE3 exhibiting a clear preference for ATC based on the VmaxATC/VmaxBTC ratio. In addition, the four PpAChEs were more sensitive to the AChE-specific inhibitor BW284C51, which acts against ATC hydrolysis, than to the BChE-specific inhibitor ISO-OMPA, which acts against BTC hydrolysis, with at least a 8.5-fold difference in IC50 values for each PpAChE. PpAChE3, PpAChE4, and PpAChE1 were more sensitive than PpAChE2 to the tested Carb insecticides, and PpAChE3 was more sensitive than the other three AChEs to the tested OP insecticides. Based on all the results, two new functional AChEs were identified from P. pseudoannulata. The differences in AChE sequence between this spider and insects enrich our knowledge of invertebrate AChE diversity, and our findings will be helpful for understanding the selectivity of insecticides between insects and natural enemy spiders.

Mutation and duplication of arthropod acetylcholinesterase: Implications for pesticide resistance and tolerance

A series of common/shared point mutations in acetylcholinesterase (AChE) confers resistance to organophosphorus and carbamate insecticides in most arthropod pests. However, the mutations associated with reduced sensitivity to insecticides usually results in the reduction of catalytic efficiency and leads to a fitness disadvantage. To compensate for the reduced catalytic activity, overexpression of neuronal AChE appears to be necessary, which is achieved by a relatively recent duplication of the AChE gene (ace) as observed in the two-spotted spider mite and other insects. Unlike the cases with overexpression of neuronal AChE, the extensive generation of soluble AChE is observed in some insects either from a distinct non-neuronal ace locus or from a single ace locus via alternative splicing. The production of soluble AChE in the fruit fly is induced by chemical stress. Soluble AChE acts as a potential bioscavenger and provides tolerance to xenobiotics, suggesting its role in chemical adaptation during evolution.

Induction of soluble AChE expression via alternative splicing by chemical stress in Drosophila melanogaster

Various molecular forms of acetylcholinesterase (AChE) have been characterized in insects. Post-translational modification is known to be a major mechanism for the molecular diversity of insect AChE. However, multiple forms of Drosophila melanogaster AChE (DmAChE) were recently suggested to be generated via alternative splicing (Kim and Lee, 2013). To confirm alternative splicing as the mechanism for generating the soluble form of DmAChE, we generated a transgenic fly strain carrying the cDNA of DmAChE gene (Dm_ace) that predominantly expressed a single transcript variant encoding the membrane-anchored dimer. 3' RACE (rapid amplification of cDNA ends) and western blotting were performed to compare Dm_ace transcript variants and DmAChE forms between wild-type and transgenic strains. Various Dm_ace transcripts and DmAChE molecular forms were observed in wild-type flies, whereas the transgenic fly predominantly expressed Dm_ace transcript variant encoding the membrane-anchored dimer. This supports alternative splicing as the major determinant in the generation of multiple forms of DmAChE. In addition, treatment with DDVP as a chemical stress induced the expression of the Dm_ace splice variant without the glycosylphosphatidylinositol anchor site in a dose-dependent manner and, accordingly, the soluble form of DmAChE in wild-type flies. In contrast, little soluble DmAChE was expressed in the transgenic fly upon exposure to DDVP. DDVP bioassays revealed that transgenic flies, which were unable to express a sufficient amount of soluble monomeric DmAChE, were more sensitive to DDVP compared to wild-type flies, suggesting that the soluble monomer may exert non-neuronal functions, such as chemical defense against xenobiotics.

Genomic structure, organization and localization of the acetylcholinesterase locus of the olive fruit fly, Bactrocera oleae

Acetylcholinesterase (AChE), encoded by the ace gene, is a key enzyme of cholinergic neurotransmission. Insensitive acetylcholinesterase (AChE) has been shown to be responsible for resistance to OPs and CBs in a number of arthropod species, including the most important pest of olives trees, the olive fruit fly Bactrocera oleae. In this paper, the organization of the B. oleae ace locus, as well as the structural and functional features of the enzyme, are determined. The organization of the gene was deduced by comparison to the ace cDNA sequence of B. oleae and the organization of the locus in Drosophila melanogaster. A similar structure between insect ace gene has been found, with conserved exon-intron positions and junction sequences. The B. oleae ace locus extends for at least 75 kb, consists of ten exons with nine introns and is mapped to division 34 of the chromosome arm IIL. Moreover, according to bioinformatic analysis, the Bo AChE exhibits all the common features of the insect AChE. Such structural and functional similarity among closely related AChE enzymes may implicate similarities in insecticide resistance mechanisms.

Utility of insects for studying human pathogens and evaluating new antimicrobial agents

Insect models, such as Galleria mellonella and Drosophila melanogaster have significant ethical, logistical, and economic advantages over mammalian models for the studies of infectious diseases. Using these models, various pathogenic microbes have been studied and many novel virulence genes have been identified. Notably, because insects are susceptible to a wide variety of human pathogens and have immune responses similar to those of mammals, they offer the opportunity to understand innate immune responses against human pathogens better. It is important to note that insect pathosystems have also offered a simple strategy to evaluate the efficacy and toxicity of many antimicrobial agents. Overall, insect models provide a rapid, inexpensive, and reliable way as complementary hosts to conventional vertebrate animal models to study pathogenesis and antimicrobial agents.

Which acetylcholinesterase functions as the main catalytic enzyme in the Class Insecta?

Most insects possess two different acetylcholinesterases (AChEs) (i.e., AChE1 and AChE2; encoded by ace1 and ace2 genes, respectively). Between the two AChEs, AChE1 has been proposed as a major catalytic enzyme based on its higher expression level and frequently observed point mutations associated with insecticide resistance. To investigate the evolutionary distribution of AChE1 and AChE2, we determined which AChE had a central catalytic function in several insect species across 18 orders. The main catalytic activity in heads was determined by native polyacrylamide gel electrophoresis in conjunction with Western blotting using AChE1- and AChE2-specific antibodies. Of the 100 insect species examined, 67 species showed higher AChE1 activity; thus, AChE1 was considered as the main catalytic enzyme. In the remaining 33 species, ranging from Palaeoptera to Hymenoptera, however, AChE2 was predominantly expressed as the main catalytic enzyme. These findings challenge the common notion that AChE1 is the only main catalytic enzyme in insects with the exception of Cyclorrhapha, and further demonstrate that the specialization of AChE2 as the main enzyme or the replacement of AChE1 function with AChE2 were rather common events, having multiple independent origins during insect evolution. It was hypothesized that the generation of multiple AChE2 isoforms by alternative splicing allowed the loss of ace1 during the process of functional replacement of AChE1 with AChE2 in Cyclorrhapha. However, the presence of AChE2 as the main catalytic enzyme in higher social Hymenoptera provides a case for the functional replacement of AChE1 with AChE2 without the loss of ace1. The current study will provide valuable insights into the evolution of AChE: which AChE has been specialized as the main catalytic enzyme and to become the main target for insecticides in different insect species.

Molecular and kinetic properties of two acetylcholinesterases from the western honey bee, Apis mellifera

We investigated the molecular and kinetic properties of two acetylcholinesterases (AmAChE1 and AmAChE2) from the Western honey bee, Apis mellifera. Western blot analysis revealed that AmAChE2 has most of catalytic activity rather than AmAChE1, further suggesting that AmAChE2 is responsible for synaptic transmission in A. mellifera, in contrast to most other insects. AmAChE2 was predominately expressed in the ganglia and head containing the central nervous system (CNS), while AmAChE1 was abundantly observed not only in the CNS but also in the peripheral nervous system/non-neuronal tissues. Both AmAChEs exist as homodimers; the monomers are covalently connected via a disulfide bond under native conditions. However, AmAChE2 was associated with the cell membrane via the glycophosphatidylinositol anchor, while AmAChE1 was present as a soluble form. The two AmAChEs were functionally expressed with a baculovirus system. Kinetic analysis revealed that AmAChE2 has approximately 2,500-fold greater catalytic efficiency toward acetylthiocholine and butyrylthiocholine than AmAChE1, supporting the synaptic function of AmAChE2. In addition, AmAChE2 likely serves as the main target of the organophosphate (OP) and carbamate (CB) insecticides as judged by the lower IC₅₀ values against AmAChE2 than against AmAChE1. When OP and CB insecticides were pre-incubated with a mixture of AmAChE1 and AmAChE2, a significant reduction in the inhibition of AmAChE2 was observed, suggesting a protective role of AmAChE1 against xenobiotics. Taken together, based on their tissue distribution pattern, molecular and kinetic properties, AmAChE2 plays a major role in synaptic transmission, while AmAChE1 has non-neuronal functions, including chemical defense.

Acetylcholinesterase of Haematobia irritans (Diptera: Muscidae): baculovirus expression, biochemical properties, and organophosphate insensitivity of the G262A mutant

This study reports the baculovirus expression and biochemical characterization of recombinant acetylcholinesterase from Haematobia irritans (L.) (rHiAChE) and the effect of the previously described G262A mutation on enzyme activity and sensitivity to selected organophosphates. The rHiAChE was confirmed to be an insect AChE2-type enzyme with substrate preference for acetylthiocholine (Km 31.3 microM) over butyrylthiocholine (Km 63.4 microM) and inhibition at high substrate concentration. Enzyme activity was strongly inhibited by eserine (2.3 x 10(-10) M), BW284c51 (3.4 x 10(-8) M), malaoxon (3.6 x 10(-9) M), and paraoxon (1.8 x 10(-7) M), and was less sensitive to the butyrylcholinesterase inhibitors ethopropazine (1.1 x 10(-6) M) and iso-OMPA (4.1 x 10(-4) M). rHiAChE containing the G262A substitution exhibited decreased substrate affinity for both acetylthiocholine (Km 40.9 microM) and butyrylthiocholine (Km 96.3 microM), and exhibited eight-fold decreased sensitivity to paraoxon, and approximately 1.5- to 3-fold decreased sensitivity to other inhibitors. The biochemical kinetics are consistent with previously reported bioassay analysis, suggesting that the G262A mutation contributes to, but is not solely responsible for observed phenotypic resistance to diazinon or other organophosphates.

Acetylcholinesterase of Stomoxys calcitrans (L.) (Diptera: Muscidae): cDNA sequence, baculovirus expression, and biochemical properties

A 2193-nucleotide cDNA encoding acetylcholinesterase (AChE) of the stable fly, Stomoxys calcitrans (L.) was sequenced and expressed in the baculovirus system. The open reading frame encoded a 91 amino acid secretion signal peptide and a 613 amino acid mature protein with 96% and 94% identity to the AChEs of Haematobia irritans (L.) and Musca domestica (L.), respectively. Structural characteristics of M. domestica, H. irritans, and Drosophila melanogaster AChEs were conserved in the S. calcitrans AChE. The recombinant enzyme was inhibited by eserine, coroxon, and paraoxon and exhibited K(m) values of 63.9μM for acetylthiocholine and 96.7μM for butyrylthiocholine, confirming its biochemical identity as an acetylcholinesterase (EC 3.1.1.7). These data will enable rapid identification and assay for mutations that reduce AChE sensitivity to organophosphate (OP) pesticides, potentially aiding resistance management efforts to prevent fixation of the mutations in pest populations.

Altered GPI modification of insect AChE improves tolerance to organophosphate insecticides

The olive fruit fly Bactrocera oleae is the most destructive and intractable pest of olives. The management of B. oleae has been based on the use of organophosphate (OP) insecticides, a practice that induced resistance. OP-resistance in the olive fly was previously shown to be associated with two mutations in the acetylcholinesterase (AChE) enzyme that, apparently, hinder the entrance of the OP into the active site. The search for additional mutations in the ace gene that encodes AChE revealed a short deletion of three glutamines (Δ3Q) from a stretch of five glutamines, in the C-terminal peptide that is normally cleaved and substituted by a GPI anchor. We verified that AChEs from B. oleae and other Dipterans are actually GPI-anchored, although this is not predicted by the "big-PI" algorithm. The Δ3Q mutation shortens the unusually long hydrophilic spacer that follows the predicted GPI attachment site and may thus improve the efficiency of GPI anchor addition. We expressed the wild type B. oleae AChE, the natural mutant Δ3Q and a constructed mutant lacking all 5 consecutive glutamines (Δ5Q) in COS cells and compared their kinetic properties. All constructs presented identical K(m) and k(cat) values, in agreement with the fact that the mutations did not affect the catalytic domain of the enzyme. In contrast, the mutants produced higher AChE activity, suggesting that a higher proportion of the precursor protein becomes GPI-anchored. An increase in the number of GPI-anchored molecules in the synaptic cleft may reduce the sensitivity to insecticides.

Functional analysis and molecular characterization of two acetylcholinesterases from the German cockroach, Blattella germanica

Two acetylcholinesterases (AChEs; BgAChE1 and BgAChE2) from Blattella germanica were functionally expressed using the baculovirus system. Kinetic analysis demonstrated that BgAChE2 had higher catalytic efficiency but lower substrate specificity than BgAChE1. With the exceptions of paraoxon and propoxur, BgAChE1 was generally less sensitive to inhibitors than BgAChE2. Western blot analysis using anti-BgAChE antibodies revealed that BgAChE1 was far more abundant in all examined tissues compared to BgAChE2, which is only present in the central nervous system. Both BgAChEs existed in dimeric form, covalently connected via a disulphide bridge under native conditions. Most fractions of BgAChE1 had a glycophosphatidylinositol (GPI) anchor, but a small fraction comprised a collagen-like tail. BgAChE2 appeared to have a collagen-GPI-fused tail. Based on the kinetic and molecular properties, tissue distribution and abundance, BgAChE1 was confirmed to play a major role in postsynaptic transmission.

Identification and characterization of a cDNA encoding the acetylcholinesterase ofHaematobia irritans(L.) (Diptera: Muscidae)

A 2217-nucleotide cDNA presumptively encoding acetylcholinesterase (AChE) of the horn fly, Haematobia irritans (L.) was sequenced. The open reading frame (ORF) encoded a 91 amino acid secretion signal peptide and a 613 amino acid mature protein with 95% identity and 98% similarity to the AChE of Musca domestica (L.). Structural features characteristic of the M. domestica and Drosophila melanogaster AChEs are conserved in the H. irritans AChE. The M. domestica and D. melanogaster AChEs are target sites for organophosphate inhibition as previously shown (Walsh et al. 2001. Biochem. J. 359: 175-181, Kozaki et al. 2002. Appl. Entomol. Zool. 37: 213-218), suggesting that this H. irritans AChE2 may be the target site for organophosphate.

Nonionic detergent phase extraction for the proteomic analysis of heart membrane proteins using label-free LC-MS

Heart diseases resulting in heart failure are among the leading causes of morbidity and mortality in the Western world and can result from either systemic disease (e.g., hypertensive heart disease, ischemic heart disease) or specific heart muscle disease (e.g., dilated cardiomyopathy/DCM). Subproteome analysis of such disease subsets affords a reduction in sample complexity, potentially revealing biomarkers of cardiac failure that would otherwise remain undiscovered in proteome wide studies. Label-free nanoscale LC-MS has been applied in this study to validate a Triton X-114-based phase enrichment method for cardiac membrane proteins. Annotation of the subcellular location combined with GRAVY score analysis indicates a clear separation between soluble and membrane-bound proteins with an enrichment of over 62% for this protein subset. LC-MS allowed confident identification and annotation of hydrophobic proteins in this control sample pilot study and demonstrates the power of the proposed technique to extract integral membrane-bound proteins. This approach should be applicable to a wider scale study of disease-associated changes in the cardiac membrane subproteome.

Synthesis of Drosophila melanogaster acetylcholinesterase gene using yeast preferred codons and its expression in Pichia pastoris

To improve the expression level of recombinant Drosophila melanogaster AChE (R-DmAChE) in Pichia pastoris, the cDNA of DmAChE was first optimized and synthesized based on the preferred codon usage of P. pastoris. The synthesized AChE cDNA without glycosylphosphatidylinositol (GPI) signal peptide sequence was then ligated to the P. pastoris expression vector, generating the plasmid pPIC9K/DmAChE. The linearized plasmid was homologously integrated into the genome of P. pastoris GS115 via electrotransformation. Finally seven transformants with high expression level of R-DmAChE activity were obtained. The highest production of R-DmAChE in shake-flask culture after 5-day induction by methanol was 718.50 units/mL, which was about three times higher than our previous expression level of native DmAChE gene in P. pastoris. Thus, these new strains with the ability to secret R-DmAChE in the medium could be used for production of R-DmAChE to decrease the cost of the enzyme expense for rapid detection of organophosphate and carbamate insecticide residues.

Existence of two membrane-bound acetylcholinesterases in the honey bee head

Two acetylcholinesterase (EC 3.1.1.7) membrane forms AChE(m1) and AChE(m2), have been characterised in the honey bee head. They can be differentiated by their ionic properties: AChE(m1) is eluted at 220 mM NaCl whereas AChE(m2) is eluted at 350 mM NaCl in anion exchange chromatography. They also present different thermal stabilities. Previous processing such as sedimentation, phase separation, and extraction procedures do not affect the presence of the two forms. Unlike AChE(m1), AChE(m2) presents reversible chromatographic elution properties, with a shift between 350 to 220 mM NaCl, depending on detergent conditions. Purification by affinity chromatography does not abolish the shift of the AChE(m2) elution. The similar chromatographic behaviour of soluble AChE strongly suggests that the occurrence of the two membrane forms is not due to the membrane anchor. The two forms have similar sensitivities to eserine and BW284C51. They exhibit similar electrophoretic mobilities and present molecular masses of 66 kDa in SDS-PAGE and a sensitivity to phosphatidylinositol-specific phospholipase C in non-denaturing conditions, thus revealing the presence of a glycosyl-phosphatidylinositol anchor. We assume that bee AChE occurs in two distinct conformational states whose AChE(m2) apparent state is reversibly modulated by the Triton X-100 detergent into AChE(m1).

Production of a novel recombinant Drosophila melanogaster acetylcholinesterase for detection of organophosphate and carbamate insecticide residues

A novel recombinant Drosophila melanogaster acetylcholinesterase (R-DmAChE) produced in Pichia pastoris was first reported in this study. We cloned the DmAChE cDNA by reverse transcription PCR with removal of the signal for glycosylphosphatidylinositol (GPI) anchor attachment and the endogenous signal peptide coding sequence, and inserted it into P. pastoris vector pPIC9K under control of the alcohol oxidase gene AOX1 promoter (5'AOX1). The expression cassette of AChE cDNA was then introduced into methylotrophic yeast GS115 and several recombinant strains expressing R-DmAChE were obtained. The secreted R-DmAChE showed high stability in neutral phosphate buffer at 4 degrees C, and its kinetic parameters were identical to those of the native DmAChE. The bimolecular rate constants of R-DmAChE to dichlorvos, aldicarb and carbaryl were ranging from three to six times higher than of native DmAChE. Within six insecticides, the R-DmAChE was more sensitive than EeAChE, NbAChE and HuAChE. For 10 widely used insecticides, the IC50 values to the R-DmAChE were much lower than those to AChEs commonly used in China. With the R-DmAChE-based assay, samples spiked with three concentrations of pesticides caused enzymatic activity inhibition with R.S.D. of 0-13.7%. These results suggest that the R-DmAChE can be useful for detection of organophosphate and carbamate insecticide residues.

Mutation of exposed hydrophobic amino acids to arginine to increase protein stability

BACKGROUND

One strategy to increase the stability of proteins is to reduce the area of water-accessible hydrophobic surface.

RESULTS

In order to test it, we replaced 14 solvent-exposed hydrophobic residues of acetylcholinesterase by arginine. The stabilities of the resulting proteins were tested using denaturation by high temperature, organic solvents, urea and by proteolytic digestion.

CONCLUSION

Although the mutational effects were rather small, this strategy proved to be successful since half of the mutants showed an increased stability. This stability may originate from the suppression of unfavorable interactions of nonpolar residues with water or from addition of new hydrogen bonds with the solvent. Other mechanisms may also contribute to the increased stability observed with some mutants. For example, introduction of a charge at the surface of the protein may provide a new coulombic interaction on the protein surface.

Exploration of the Drosophila acetylcholinesterase substrate activation site using a reversible inhibitor (Triton X-100) and mutated enzymes

Cholinesterases are activated at low substrate concentration, and this is followed by inhibition as the level of substrate increases. However, one of these two components is sometimes lacking. In Drosophila acetylcholinesterase, the two phases are present, allowing both phenomena to be studied. Several kinetic schemes can explain this complex kinetic behavior. Among them, one model assumes that activation results from the binding of a substrate molecule to a non-productive site affecting the entrance of a substrate molecule into the active site. To test this hypothesis, we looked for an inhibitor competitive for activation and we found Triton X-100. Using organophosphates or carbamates as hemisubstrates, we showed that Triton X-100 inhibits or increases phosphorylation or carbamoylation of the enzyme. In vitro mutagenesis of the residues lining the active site gorge allowed us to locate the Triton X-100 binding site at the rim of the gorge with glutamate 107 playing the major role. These results led to the hypothesis that substrate binding at this site affects the entrance of another substrate molecule into the active site cleft.

Nippostrongylus brasiliensis: characterisation of a somatic amphiphilic acetylcholinesterase with properties distinct from the secreted enzymes

We have previously determined that Nippostrongylus brasiliensis secretes three monomeric nonamphiphilic (G1na) variants of acetylcholinesterase (AChE) with broadly similar properties. In this study we have examined AChE expression in somatic extracts of N. brasiliensis and report the identification of an additional enzyme which is not secreted. The enzyme was resolved by sucrose density gradient centrifugation with a sedimentation coefficient of 10.2 S which was shifted to 9.4 S in the presence of Triton X-100, identifying the enzyme as a tetrameric amphiphilic (G4a) form. The amphiphilic properties of this enzyme were confirmed by charge-shift electrophoresis, in which migration was accelerated by interaction with sodium deoxycholate. The enzyme showed low activity with butyrylthiocholine, and a Michaelis constant of 91 +/- 13 microM for acetylthiocholine was determined. It was highly sensitive to the AChE-specific inhibitor bis (4-allyldimethylammoniumphenyl)pentan-3-one dibromide, with an IC50 of 6.5 +/- 0.4 microM, but was also inhibited by the butyrylcholinesterase-specific inhibitor tetramonoisopropylpyrophosphortetramide, albeit with a higher IC50 of 46.5 +/- 6.1 microM. This enzyme can therefore be distinguished from the secreted AChEs by its amphiphilic properties, sedimentation in sucrose gradients, and sensitivity to cholinesterase inhibitors.

Stabilization of recombinant Drosophila acetylcholinesterase

The uses of pure and stable acetylcholinesterase can range from simple basic research to applications in environment quality assessment. In order to satisfy some of these needs its recombinant expression is routinely performed. Affinity-purified recombinant Drosophila melanogaster acetylcholinesterase proved to be instable; an apparent cause of this seemed to be the presence of contaminants with protease activity as evidenced by SDS-PAGE. The elimination of these accompanying products was achieved by anion-exchange, hydrophobic interaction, and cibacron blue affinity chromatography applied downstream from procainamide affinity chromatography. The utilization of a parallel affinity acting via an engineered histidine tail permitted the elimination of the copurified proteases as well. Despite the elimination of the contaminants, the apparently pure extracts were still unstable. It is shown that such instability can be counterbalanced by provoking protein-protein interactions, either between enzyme molecules or with other molecules such as bovine serum albumin. Another way to reduce instability is the addition of a reversible inhibitor or polyethylene glycol 3350.

Drosophila acetylcholinesterase: effect of post-translational [correction of post-traductional] modifications on the production in the baculovirus system and substrate metabolization

Acetylcholinesterase cDNAs from Drosophila melanogaster modified on its primary sequence were cloned into baculovirus and were expressed in Sf9 cells with the aim to identify a mutant form that produces the enzyme at a high level. Directed mutagenesis was used in order to independently knockout different sites of post-translational modifications: exchange of the C-terminal hydrophobic peptide for a glycolipid molecule, dimerization by disulfide bridge, N-linked glycosylation at the five accessible sites, and subunit formation by proteolytic cleavage of a hydrophilic peptide found in the precursor. Another mutation involved the elimination of a free cysteine in the mature protein. All mutations involving post-translational modifications resulted in lower recoveries, suggesting that they are useful for maintaining high amounts of protein in the synapse. By contrast, elimination of a free cysteine in the mature protein permitted an increase in the level of production of the enzyme. These mutations did not affect specific activity of the enzyme at substrate concentrations ranging from 3 microM to 200 mM, suggesting that activation and inhibition of the enzyme activity does not originate from a polymorphism in post-translational modifications.

Purification and properties of monomeric (G1) forms of acetylcholinesterase secreted by Nippostrongylus brasiliensis

Acetylcholinesterase (AChE) activity secreted by Nippostrongylus brasiliensis was resolved by sucrose density centrifugation and gel permeation chromatography in single peaks estimated at 4.3 S and 60-85 kDa, respectively. Sedimentation was unaffected by the inclusion of detergent. AChE was purified by affinity chromatography on 9-[Nbeta-(epsilon-aminocaproyl)-beta-aminopropylamino]-acridinium bromide hydrobromide-coupled sepharose 4B. Three forms of the enzyme (A, B and C) were distinguished by non-denaturating polyacrylamide gel electrophoresis, and displayed apparent masses of 74, 69 and 71 kDa respectively when resolved by SDS-PAGE. All three isoforms showed a preference for acetylthiocholine (ASCh) as substrate. They were highly sensitive to inhibition by the AChE-specific inhibitor bis(4-allyldimethylammoniumphenyl)pentan-3-one dibromide, with inhibitor concentration reducing initial activity by 50% (IC50) between 0.1 and 0.8 microM, but activity was unaffected by tetramonoisopropylpyrophosphortetramide (iso-OMPA) at concentrations up to 10 mM. We conclude that the secreted enzymes are authentic AChEs of hydrophilic monomeric (G1) form and broadly similar properties, but which can be distinguished by molecular mass, inhibitor sensitivities and the degree of excess substrate inhibition.

Cholinesterases from the common oyster (Crassostrea gigas). Evidence for the presence of a soluble acetylcholinesterase insensitive to organophosphate and carbamate inhibitors

Marine bivalves such as oysters and mussels are widely used as bioindicators of contamination in the monitoring of pollutant effects. As filter feeders, these species are known to be good general indicators of chemical contamination. However, the efficient use of decreased acetylcholinesterase activity in the oyster as a biomarker of exposure to neurotoxic compounds requires a definition of the different types of cholinesterases coexisting in this mollusk. This study reports the partial purification, separation and characterization of two cholinesterases extracted from the oyster Crassostrea gigas. Differences in apparent molecular weight, type of glycosylation and hydrophobicity, and sensitivity to inhibitors suggest that they are encoded by two different genes. 'A' cholinesterase (apparent molecular weight 200 kDa) is anchored to the membrane via a glycolipid, is not glycosylated but sensitive to organophosphate and carbamate inhibitors. 'B' cholinesterase (molecular weight 330 kDa) is hydrophilic, glycosylated and highly resistant to organophosphate and carbamate inhibitors. The kinetic properties of these two cholinesterases were compared with those of other invertebrate cholinesterases. The presence of a cholinesterase insensitive to insecticides suggests that a significant improvement in the use of oyster cholinesterases as biomarkers of pollutant effects could be achieved by simple separation of the two forms.

Replacement of the glycoinositol phospholipid anchor of Drosophila acetylcholinesterase with a transmembrane domain does not alter sorting in neurons and epithelia but results in behavioral defects

Drosophila has a single glycoinositol phospholipid (GPI)-anchored form of acetylcholinesterase (AChE) encoded by the Ace locus. To assess the role that GPI plays in the physiology, of AChE, we have replaced the wild-type GPI-AChE with a chimeric transmembrane form (TM-AChE) in the nervous system of the fly. Ace null alleles provided a genetic background completely lacking in endogenous GPI-AChE, and Ace minigene P transposon constructs were used to express both GPI- and TM-AChE forms in the tissues where AChE is normally expressed. Control experiments with the GPI-AChE minigene demonstrated a threshold between 9 and 12% of normal AChE activity for adult viability. Ace mutant flies were rescued by GPI-AChE minigene lines that expressed 12-40% of normal activity and were essentially unchanged from wild-type flies in behavior. TM-AChE minigene lines were able to rescue Ace null alleles, although with a slightly higher threshold than that for GPI-AChE. Although rescued flies expressing GPI-AChE at a level of 12% of normal activity were viable, flies expressing 13-16% of normal activity from the TM-AChE transgene died shortly after eclosion. Flies expressing TM-AChE at about 30% of normal levels were essentially unchanged from wild-type flies in gross behavior but had a reduced lifespan secondary to subtle coordination defects. These flies also showed reduced locomotor activity and performed poorly in a grooming assay. However, light level and electron microscopic immunocytochemistry showed no differences in the localization of GPI- and TM-AChE. Furthermore, endogenous and ectopic-induced expression of both AChEs in epithelial tissues of the adult and embryo, respectively, showed that they were sorted identically. Most epithelial cells sorted GPI- and TM-AChE to the apical surface, but cuticle-secreting epithelia sorted both proteins basolaterally. Our data suggest that rather than having a primary role in protein sorting, the GPI anchor or AChE plays some other more subtle cellular role in neuronal physiology.

Acetylcholinesterase in tentacles of Octopus vulgaris (Cephalopoda). Histochemical localization and characterization of a specific high salt-soluble and heparin-soluble fraction of globular forms

Transverse sections of Octopus tentacles were stained for acetylcholinesterase (AChE) activity. An intense staining, that was suppressed by preincubation in 10(-5) M eserine, was detected in a number of neuronal cells, nerve fibres and neuromuscular junctions of intrinsic muscles of the arm. Octopus acetylcholinesterase was found as two molecular forms: an amphiphilic dimeric form (G2) sensitive to phosphatidylinositol phospholipase C and a hydrophilic tetrameric (G4) form. Sequential solubilization revealed that a significant portion of both G2 and G4 forms was recovered only in a high salt-soluble fraction (1 M NaCl, no detergent), Heparin (2 mg/ml) was able to solubilize G2 and G4 forms with the same efficiency than 1 M NaCl. The solubilizing effect of heparin was concentration-dependent and was reduced by protamine (2 mg/ml). This suggests that heparin operates through the dissociation of ionic interactions existing in situ between globular forms of AChE and cellular or extracellular polyanionic components. Interaction of AChE molecular forms with heparin has been reported so far in only a few instances and its physiological meaning is uncertain. G2 and G4 forms, interacting or not with heparin, all belong to a single pharmacological class of AChE. This suggests the existence of a single AChE gene. Amphiphilic and hydrophilic subunits thus likely result either from the processing of a single AChE transcript by alternative splicing (as in vertebrate AChE) or from a post-translation modification of a single catalytic peptide.

Hydrophilic and amphiphilic forms of Drosophila choline acetyltransferase are encoded by a single mRNA

We have previously shown that the enzyme choline-O-acetyltransferase (ChAT) exists in a hydrophilic and an amphiphilic form in Drosophila head. A complementary DNA clone of 4.2 kb containing the entire coding region of ChAT was isolated from a cDNA library of Drosophila heads. The cDNA was subcloned in an expression vector and injected into the nucleus of Xenopus oocytes. Injected oocytes expressed high levels of ChAT activity. This activity was inhibited by bromoacetylcholine, a specific inhibitor of the enzyme. In the present study the non-ionic detergent Triton X-114 was used to analyse whether the expression of hydrophilic and amphiphilic ChAT was or was not directed by a single cDNA. The two forms of ChAT were found to be synthesized in injected oocytes. Approximately 9% of the recombinant enzyme partitioned as amphiphilic activity. This value was similar to that found for native amphiphilic ChAT in Drosophila heads. Sedimentation in sucrose gradients of amphiphilic enzyme was found to be influenced by the type of detergent present in the gradient whereas this was not the case for hydrophilic ChAT. Hydrophilic and amphiphilic enzyme activities differed in some of their biochemical properties. Amphiphilic ChAT was less sensitive to inhibition by the product acetylcholine than was hydrophilic ChAT. Moreover, amphiphilic ChAT was found to be more resistant than hydrophilic ChAT to heat inactivation at 45 degrees C. These properties were observed for the native as well as for recombinant ChAT. These results demonstrate that the hydrophilic and amphiphilic forms of ChAT are derived from one mRNA.

The molecular forms of acetylcholinesterase from Necator americanus (Nematoda), a hookworm parasite of the human intestine

Necator americanus (Nematoda: Strongyloidea), a human hookworm parasite, is known to release considerable amounts of acetylcholinesterase (AChE) [Pritchard, D. I., Leggett K. V., Rogan, M. T., McKean, P. G. & Brown, A. (1991) Necator americanus secretory acetylcholinesterase and its purification from excretory/secretory products by affinity chromatography, Parasite Immunol. 13, 187-199]. The present study deals with AChE activity recovered in sequential somatic extracts, and excretory/secretory products, of the adult stage of the parasite. 97% of AChE was extractable in low-salt and high-salt detergent-free buffers, and only 3% was solubilised by a further extraction in the presence of Triton X-100. AChE in all three extracts was affected by the AChE inhibitors eserine, bis(4-allyldimethylammoniumphenyl)pentan-3-one dibromide and edrophonium chloride, but was resistant to the effects of tetramonoisopropylpyrophosphortetramide, a butyrylcholinesterase inhibitor. Sucrose density centrifugation revealed that AChE in all somatic extracts (low-salt, high-salt and detergent) resolved almost exclusively as a single peak between 6.9-7.5 S, while excretory/secretory products resolved at 8.2 S. These values are all compatible with dimers of catalytic subunits and no evidence was found for the presence of higher oligomers such as asymmetric forms. The only sample to show a shift in sedimentation following the inclusion of detergent (Triton X-100, Brij 96) in the gradient was a component of the detergent-soluble extract, indicating the existence of a minor amphiphilic form. In low-salt-soluble and high-salt-soluble extracts, AChE was solubilised as a hydrophilic globular form, probably a dimeric G2. The analysis of diisopropylfluorophosphate-labelled extracts by SDS/PAGE, and unlabelled extracts by immunoblotting using a polyvalent antiserum to N. americanus AChE, indicated that the AChE isolated in each extract was biochemically and immunologically similar. The banding patterns obtained were comparable to that seen when purified AChE was analysed by SDS/PAGE and immunoblotted. This suggests that the basic catalytic subunit has a mass of 66-70 kDa with the active site being located in a 30-kDa domain. All experimental data indicate the existence of only one AChE class in Necator homologous to AChE of class B from Caenorhabditis elegans. The solubility characteristics and globular nature of this hookworm AChE suggest that its major function is as an excretory or secretory product. This again raises the question of the true biological function of this 'non-cholinergenic' nematode secretion.

Brain myelin-bound Zn(2+)-glycerophosphocholine cholinephosphodiesterase is a glycosylphosphatidylinositol-anchored enzyme of two different molecular forms

An Zn(2+)-GPC cholinephosphodiesterase activity, which is present more predominantly in myelin than in microsome or cytosol, has been examined using rho-nitrophenylphosphocholine as a substrate. In the solubilization of enzyme activity from myelin membranes, lysolecithin was found to be more effective than Triton X-100 or deoxycholate. Especially, the myelin-bound phosphodiesterase was suggested to be a glycosylphosphatidyl-inositol-anchored protein, based on solubilization by B. cereus phospholipase C and Triton X-114 phase separation. Interestingly, it was found that while phospholipase C-solubilized enzyme, a hydrophilic protein, was associable with Concanavalin A column, detergent-solubilized amphiphilic form of enzyme was not. Either detergent extract or cytosol was observed to contain both amphiphilic form and hydrophilic one. In CM-sephadex chromatography, the soluble hydrophilic phosphodiesterase was observed to be separatable into two forms of enzyme. In comparative studies, both forms of phosphodiesterase showed much similarity in substrate specificity, optimum pH, Km value and Zn2+ requirement, although they differed in charge property and molecular weight.

Evolutionary genetics of Drosophila esterases

Over 30 carboxylester hydrolases have been identified in D. melanogaster. Most are classified as acetyl, carboxyl or cholinesterases. Sequence similarities among most of the carboxyl and all the cholinesterases so far characterised from D. melanogaster and other eukaryotes justify recognition of a carboxyl/cholinesterase multigene family. This family shows minimal sequence similarities with other esterases but crystallographic data for a few non-drosophilid enzymes show that the family shares a distinctive overall structure with some other carboxyl and aryl esterases, so they are all put in one superfamily of/beta hydrolases. Fifteen esterase genes have been mapped in D. melanogaster and twelve are clustered at two chromosomal sites. The constitution of each cluster varies across Drosophila species but two carboxyl esterases in one cluster are sufficiently conserved that their homologues can be identified among enzymes conferring insecticide resistance in other Diptera. Sequence differences between two other esterases, the EST6 carboxyl esterase and acetylcholinesterase, have been interpreted against the consensus super-secondary structure for the carboxyl/cholinesterase multigene family; their sequence differences are widely dispersed across the structure and include substantial divergence in substrate binding sites and the active site gorge. This also applies when EST6 is compared across species where differences in its expression indicate a difference in function. However, comparisons within and among species where EST6 expression is conserved show that many aspects of the predicted super-secondary structure are tightly conserved. Two notable exceptions are a pair of polymorphisms in the substrate binding site of the enzyme in D. melanogaster. These polymorphisms are associated with differences in substrate interactions in vitro and demographic data indicate that the alternative forms are not selectively equivalent in vivo.

Characterization of an antibody to the cross-reacting determinant of the glycosyl-phosphatidylinositol anchor of human membrane dipeptidase

A polyclonal antiserum raised to the phospholipase C-solubilized form of membrane dipeptidase (EC 3.4.13.11) purified from human kidney was found to cross-react with unrelated trypanosomal and porcine glycosyl-phosphatidylinositol anchored proteins. Those antibodies recognising the cross-reacting determinant (CRD) were isolated by chromatography on a column of immobilized phospholipase C-solubilized porcine aminopeptidase P (EC 3.4.11.9), and the epitopes involved in the recognition were then characterized by immunoelectrophoretic blot analysis and by a competitive ELISA. The phospholipase C-solubilized forms of human and porcine membrane dipeptidase, porcine aminopeptidase P and trypanosome variant surface glycoprotein were recognised by the anti-CRD antiserum, and this recognition was abolished by prior treatment of the proteins with either mild acid or nitrous acid. In contrast, the detergent-solubilized, membrane-forms of human and porcine membrane dipeptidase were not recognised. Of a range of components of the glycosyl-phosphatidylinositol anchor, only inositol 1,2-cyclic monophosphate and the insulin-mimetic disaccharide, glucosaminyl-1,6-inositol 1,2-cyclic monophosphate, inhibited in the micromolar range the binding of the anti-CRD antiserum to immobilized porcine aminopeptidase P. These results indicate that the major epitope recognised by this anti-CRD antiserum is the inositol 1,2-cyclic monophosphate formed on phospholipase C cleavage of the glycosyl-phosphatidylinositol anchor.

Acetylcholinesterases of the nematode Steinernema carpocapsae. Characterization of two types of amphiphilic forms differing in their mode of membrane association

We analyzed the molecular forms of acetylcholinesterase (AChE) in the nematode Steinernema carpocapsae. Two major AChEs are involved in acetylcholine hydrolysis. The first class of AChE is highly sensitive to eserine (IC50 = 0.05 microM). The corresponding molecular forms are: an amphiphilic 14S form converted into a hydrophilic 14.5S form by mild proteolysis and two hydrophilic 12S and 7S forms. Reduction of the amphiphilic 14S form with 10 mM dithiothreitol produces hydrophilic 7S and 4S forms, indicating that it is an oligomer of hydrophilic catalytic subunits linked by disulfide bond(s) to a hydrophobic structural element that confers the amphiphilicity to the complex. Sedimentation coefficients suggest that 4S, 7S, 12S forms correspond to hydrophilic monomer, dimer, tetramer and that the 14S form is also a tetramer linked to one structural element. The second class of AChE is less sensitive to eserine (IC50 = 0.1 mM). Corresponding molecular forms are hydrophilic and amphiphilic 4S forms (monomers) and a major amphiphilic 7S form converted into a hydrophilic dimer by Bacillus thuringiensis phosphatidylinositol-specific phospholipase C. This amphiphilic 7S form thus possesses a glycolipid anchor. It appears that Steinernema (a very primitive invertebrate) presents AChEs with two types of membrane association that closely resemble those described for amphiphilic G2 and G4 forms of AChE in more evolved animals.

Butyrylcholinesterase from chicken brain is smaller than that from serum: its purification, glycosylation, and membrane association

Applying a new four-step isolation procedure, we have purified butyrylcholinesterase (BChE) from chicken serum to homogeneity with more than 250 U/mg specific activity. The serum enzyme was used for producing monoclonal antibodies. These BChE-specific also recognize BChE from brain, and thus enabled us to isolate the enzymes from embryonic and adult brain that occur only in minute amounts. More than 50% of the brain BChE is membrane-bound. The catalytic and inhibition properties of brain BChE are similar to those of serum BChE. However on sodium dodecyl sulfate-polyacrylamide gel electrophoresis, the serum enzyme is represented by a double-band of 79/82 kDa, whereas the brain enzyme has a size of 74 kDa. Limited digestion of the serum and brain preparations by V8-protease leads to similar peptide patterns. Enzymatic deglycosylation shows that their core proteins consist of 59-kDa subunits and that the different molecular weights are due to different glycosylation patterns. The differently sized glycosylation parts of brain and serum BChE may indicate that they subserve different functions. Furthermore, the membrane-bound brain BChE can be solubilized by Pronase or protease K, but not by phosphatidylinositol-specific phospholipase C.

Purification and characterization of the higher plant enzyme L-canaline reductase

A newly discovered enzyme, L-canaline reductase (NADPH:L-canaline oxidoreductase, EC 1.6.6-), has been isolated and purified from 10-day-old leaves of the jack bean Canavalia ensiformis (Leguminosae). This higher plant is representative of a large number of legumes that synthesize L-canavanine, an important nitrogen-storing nonprotein amino acid. Canavanine-storing legumes contain arginase, which hydrolyzes L-canavanine to form the toxic metabolite L-canaline. Canaline reductase, having a mass of approximately 167 kDa and composed of 82-kDa dimers, catalyzes a NADPH-dependent reductive cleavage of L-canaline to L-homoserine and ammonia. This is the only enzyme known to use reduced NADP to cleave an O-N bond. Canaline reductase performs at least three important functions for canavanine-synthesizing legumes. First, it detoxifies canaline. Second, it increases by one-half the overall yield of ammoniacal nitrogen released from canavanine. Third, it permits the carbon skeleton of canavanine, a secondary plant metabolite, to support vital primary metabolic reactions.

Drosophila acetylcholinesterase. Expression of a functional precursor in Xenopus oocytes

In insects, acetylcholinesterase is mainly found in the central nervous system. It is expressed in the synapse where it hydrolyzes the neurotransmitter acetylcholine. Maturation of this protein involves several post-translational modifications. The precursor polypeptide is cut at three sites; the N-terminal signal peptide is removed, the C-terminal hydrophobic polypeptide is clipped off and replaced by a glycolipid anchor and the resulting peptide is cut into two polypeptides, corresponding to active subunits. Two of these active subunits are associated to form the final active glycosylated protein. We have expressed the protein via microinjection of an expression vector into Xenopus oocyte nuclei. When the complete cDNA is injected, the acetylcholinesterase formed is biochemically similar to the Drosophila-head acetylcholinesterase. However, the hydrophobic C-terminal peptide is not replaced by a glycolipid anchor. As a consequence, the enzyme is no longer externalized, the proteolytic cutting of the main peptide does not occur and a new polymerization form occurs. Although incompletely processed, this protein is enzymatically active. When a cDNA lacking the coding region of the C-terminal hydrophobic peptide is injected, the resulting acetylcholinesterase is hydrophilic, cleaved into two subunits and secreted into the incubation medium free of contaminants.

Minigene rescues acetylcholinesterase lethal mutations in Drosophila melanogaster

The gene encoding acetylcholinesterase in Drosophila melanogaster is over 34,000 base-pairs long. We have constructed a 5800 base-pair minigene containing 1500 base-pairs of genomic sequence upstream from the transcription start spliced to the coding sequence, but lacking the nine introns. After germline genetic transformation, this minigene rescues acetylcholinesterase lethal mutants. Tissue-specific distribution appears normal. This allows us to test site-directed mutations of acetylcholinesterase. In a first effort, deletion of most of the unusual 1000 bases leader and its intriguing short open reading frames showed no effect on gene expression. The way is open to study in vivo the structure-function relationships of acetylcholinesterase and insecticide resistance.

Glycosyl-phosphatidylinositol anchored acetylcholinesterase as substrate for phosphatidylinositol-specific phospholipase C from Bacillus cereus

We investigated the enzymatic properties of phosphatidylinositol-specific phospholipase C (PI-PLC) from Bacillus cereus towards glycosyl-phosphatidylinositol anchored acetylcholinesterase (AChE) from bovine erythrocytes and Torpedo electric organ as substrate. The conversion of membrane from AChE to soluble AChE by PI-PLC depended on the presence of a detergent and of phosphatidylcholine. In presence of mixed micelles containing Triton X-100 (0.05%) and phosphatidylcholine (0.5 mg/ml) the rate of AChE conversion was about 3 times higher than in presence of Triton X-100 alone. Furthermore, inhibition of PI-PLC occurring at Triton X-100 concentrations higher than 0.01% could be prevented by addition of phosphatidylcholine. Ca2+, Mg2+ and sodium chloride had no effect on PI-PLC activity in presence of phosphatidylcholine and Triton X-100, whereas in presence of Triton X-100 alone sodium chloride largely increased the rate of AChE conversion. Determination of kinetic parameters with three different substrates gave Km-values of 7 microM, 17 microM and 2 mM and Vmax-values of 0.095 microM.min-1, 0.325 microM.min-1 and 56 microM.min-1 for Torpedo AChE, bovine erythrocyte AChE and phosphatidylinositol, respectively. The low Km-values for both forms of AChE indicated that PI-PLC not only recognized the phosphatidylinositol moiety of the anchor but also other components thereof.